1. Field of the Invention
This invention relates to a push-pull inverter used, for example, as a driver for cold-cathode discharge tube, hot-cathode discharge tube, etc.
2. Background Art
FIG. 8 illustrates a conventional embodiment of the push-pull inverter constructed as the driver for a fluorescent lamp 11.
This inverter is a push-pull circuit substantially comprising a booster transformer 12, transistors 13, 14 for switching function, a capacitor 15 for a resonance circuit and a choke coil 16.
Upon closure of a source switch 17, a transistor 18 serving as a power supply switch is turned on so as to supply the push-pull circuit with DC power from a DC source 19.
Then, the transistors 13, 14 are supplied through resistors 20, 21, respectively, with their base current. Though both the transistors 13, 14 are consequently switched to their conductive states, they are different from each other in transistor characteristic and circuit arrangement so that one of them becomes more positively conductive than the other and said one transistor is turned on earlier than the other.
For example, when the transistor 13 is turned on earlier than the transistor 14, the current supplied from the DC source 19 passes through the choke coil 16 into a primary coil 12P (more strictly to say, a primary coil section 12P.sub.1 on one side) of the transformer 12 and this primary coil 12P generates thereacross the voltage of a direction as indicated by a solid line arrow in FIG. 8, resulting in that a collector potential of the transistor 13 becomes lower than a collector potential of the transistor 14.
Since, at this time point, a tertiary coil 12F generates thereacross the voltage of a direction as indicated by a solid line arrow in FIG. 8, the base of the transistor 13 is affected by a posttive feedback effect and the collector current of this transistor 13 rapidly increases. Thereupon, a secondary coil 12S generates thereacross the inductive voltage of a direction as indicated by a solid line arrow in FIG. 8, which initiates lighting of the fluorescent lamp 11.
Since increase in the current flowing through the transistor 13 is suppressed at a saturation point which depends upon a base current as well as an amplification degree, the voltage of a direction as indicated by a broken line arrow in FIG. 8 is generated across the primary coil 12P of the transformer 12 and the transistor 13 is switched from ON to OFF while the transistor 14 is switched from OFF to ON, as said increase in the current is reduced
When the transistor 14 is turned on, the voltage of a direction as indicated by a broken line arrow in FIG. 8 is generated across the tertiary coil 12F and consequently the base of the transistor 14 is affected by the positive feedback and the current flowing through the transistor 14 increases, resulting in that the inductive voltage of a direction as indicated by a broken line arrow in FIG. 8 is generated across the secondary coil 12S, which maintains lighting of the fluorescent lamp 11.
Thereafter alternate turning-on of the transistors 13, 14 repeatedly occurs in the same manner as has been mentioned above, generating a high AC voltage across the secondary coil 12S.
The primary coil 12P of the transformer 12 forms, in cooperation with the capacitor 15, a resonance circuit. Under a resonance voltage of this resonance circuit, the secondary coil 12S outputs an AC voltage V.sub.9 and an alternating current I.sub.9 as shown by FIG. 9 and the fluorescent lamp 11 has a load voltage V.sub.10 and a load current I.sub.10 as shown by FIG. 10.
Referring to FIG. 8, reference numeral 22 designates a capacitor for stabilized source voltage and reference numeral 23 designates a capacitor for stabilized operation.
In the inverter of prior art as has been described above, the transformer 12 must be provided with the tertiary coil 12F for feedback
Consequently, at least a step of winding said tertiary coil 12F and a step of soldering opposite ends of this coil to respective terminal pins are additionally required. This is undesirable for improvement in efficiency with which the transformer is produced
Since the above-mentioned transformer 12 is small-sized, provision of said tertiary coil 12F requires the terminal pins exclusively for this coil, which makes miniaturization of the transformer 12 difficult.
Specifically, said transformer 12 requires seven terminal pins in total, comprising three terminal pins for the primary coil 12P, two terminal pins for the secondary coil 12S and two terminal pins for the tertiary coil 12F.
One of the terminal pins to be used as the terminal on the high voltage side of the secondary coil 12S should be spaced from the other terminal pins and fixed to one flange of a bobbin on which the coil is wound (a bobbin provided on opposite sides of the wounded area with flanges) while the other terminal pins are fixed to the other flange. In other words, said other flange are provided with six terminal pins fixed thereto at suitable angular intervals and, in consequence, the bobbin should have a relatively bulky configuration, making a desired miniaturization of the transformer difficult.
It should be understood that, in FIG. 8, the terminal pin to which a beginning end of the secondary coil 12S is connected serves as the terminal pin on the high voltage side.
Additionally, in the above-mentioned inverter of prior art, the primary coil 12P of the transformer 12 must include the capacitor 15 for the resonance circuit. Resonance current flowing through the resonance circuit causes the input current from the DC source 19 and, therefore, heating of the transformer 12 to increase. The problem of such heating becomes serious as the transformer is miniaturized.